1. Field of the Invention
The present invention relates to wireless communication systems, and more particularly, to a wireless communication system, a terminal, and a message sending method in which, when the terminal sends a message to a communicating terminal, it performs ranging to measure the distance with the communicating terminal and checks that the communicating terminal is located within a predetermined range. The invention also pertains to a program for allowing the terminal to execute the message sending method.
2. Description of the Related Art
In a typical communication system, an attribute certificate is issued when a terminal is connected to a network, and by this attribute certificate, an access right of the terminal to connect to the network is certified. In a wireless communication system, however, since terminals are wirelessly connected to a network, wiretapping or posing is more encouraged than in a wired communication system. Accordingly, when issuing an attribute certificate, access from unspecified terminals must be restricted to reliably authenticate a communicating terminal.
Thus, when authenticating a communicating terminal, certificates are exchanged via physical media, for example, portable storage media, or via short-distance wireless media, for example, non-contact integrated circuit (IC) cards.
Additionally, a technique for improving the security by switching the transmission power or the modulation method between when authenticating a communicating terminal and when performing normal data communication has been proposed. For example, Japanese Unexamined Patent Application Publication No. 2000-295658 (FIG. 1) discloses the following technique. When performing wireless communication, the transmission power is progressively increased until there is a response from a communicating device, and communication is then performed for authenticating the communicating device at the level of transmission power when a response from the communicating device is returned. With this technique, the interception by the third party can be prevented.
In the method using physical media, however, although terminals are provided with wireless media, the user has to manually attach and remove the physical media, thereby decreasing the ease of operation. In the method using short-distance wireless media, short-distance wireless media should be sometimes used in terminals, which serve as intermediate-distance wireless media, thereby increasing the complexity of a system. In the method switching the transmission power or the modulation method, a switching control system becomes complicated, and also, retry should be made many times due to the influence of the communication quality when conducting authentication.